In recent years, with the rapid spread of small electronics such as a mobile telephone and a notebook-sized personal computer, demand for a non-aqueous electrolyte secondary battery which is used as a chargeable and dischargeable power supply has been rapidly increased.
As a positive electrode active material for a non-aqueous electrolyte secondary battery, a lithium-nickel composite oxide represented by lithium nickel dioxide (LiNiO2), lithium-manganese composite oxide represented by lithium manganese dioxide (LiMnO2) and the like have been widely used as well as lithium-cobalt composite oxide represented by lithium cobalt dioxide (LiCoO2).
However, there are some defects in the lithium cobalt dioxide, such that the lithium cobalt dioxide is expensive because its reserve in the earth is a little, and that the lithium cobalt dioxide contains cobalt which is unstable in supply and has a highly fluctuating price range as a major component. Therefore, there have been remarked a lithium-nickel composite oxide containing relatively inexpensive nickel as a major component and lithium-manganese composite oxide containing relatively inexpensive manganese as a major component from the viewpoint of reduction in cost.
The lithium manganese dioxide is superior in thermal stability to lithium cobalt dioxide. However, the lithium manganese dioxide has some problems in practical use in a battery, because its charge and discharge capacity is much smaller than that of the other materials, and its charge and discharge cycle characteristic showing life of a battery is also much shorter than the other materials. On the other hand, since the lithium nickel dioxide has a charge and discharge capacity greater than the lithium cobalt dioxide, the lithium nickel dioxide has been expected to be used as a positive electrode active material which enables to produce an inexpensive battery having a high energy density.
This lithium nickel oxide has been usually prepared by mixing a lithium compound with a nickel compound such as nickel hydroxide or nickel oxyhydroxide, and calcining the resulting mixture. The form of the lithium nickel oxide is a powder in which primary particles are mono-dispersed, or a powder of secondary particles formed by aggregation of primary particles, and having spaces between the primary particles. However, both powders have some defects such that the powders are inferior in thermal stability under the condition of charging to the lithium cobalt dioxide.
In other words, since pure lithium nickel dioxide has defects in thermal stability, charge and discharge cycle characteristics and the like, the lithium nickel dioxide cannot be used in a practical battery. This is based on that the lithium nickel dioxide is inferior in stability of a crystal structure under a charging condition to the lithium cobalt dioxide.
Therefore, in order to stabilize a crystal structure under the condition that lithium is eliminated from the crystal structure in a charging process, and to obtain a lithium-nickel composite oxide having favorable thermal stability and charge and discharge cycle characteristics as a positive electrode active material, there has been generally carried out replacement of a part of nickel contained in the lithium-nickel composite oxide with other element.
For example, Patent Literature 1 proposes a non-aqueous battery in which a compound represented by the formula: LiaMbNicCodOe in which M is at least one metal selected from the group consisting of Al, Mn, Sn, In, Fe, V, Cu, Mg, Ti, Zn and Mo, 0<a<1.3, 0.02≤b≤0.5, 0.02≤d/c+d≤0.9, 1.8<e<2.2 and b+c+d=1 is used as a positive electrode active material.
In addition, as a process for improving thermal stability of the lithium-nickel composite oxide, a process for washing lithium nickel dioxide with water after calcining has been developed.
For example, Patent Literature 2 proposes a process for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, which includes calcining nickel hydroxide or nickel oxyhydroxide at a temperature of 600° C. to 1100° C. in the air to prepare nickel oxide, mixing the resulting nickel oxide with a lithium compound, subsequently calcining the resulting mixture in an oxygen atmosphere at a maximum temperature of 650° C. to 850° C., washing the resulting calcined powder with water within a period of time which satisfies the relation of A≤B/40 in which A is a period of time for washing with water of which unit is minute, and B is a concentration of slurry of the calcined powder of which unit is g/L, and thereafter carrying out filtration and drying of the calcined powder.
However, when a part of nickel included in the lithium-nickel composite oxide is substituted with other element in a large amount (in other words, under a condition of a lower content of nickel), although thermal stability of the lithium-nickel composite oxide is improved, battery capacity is lowered. On the other hand, when a part of nickel included in the lithium-nickel composite oxide is substituted with other element in a small amount (in other words, under a condition of a higher content of nickel) in order to prevent lowering in battery capacity, thermal stability of the lithium-nickel composite oxide cannot be sufficiently improved. Moreover, when the content of nickel is increased, there also arise some problems such that cation mixing easily occurs in calcining, and therefore synthesis of the lithium-nickel composite oxide becomes difficult.
In addition, when lithium nickelate which is washed with water after calcining is used in a non-aqueous electrolyte secondary battery, it is thought that a positive electrode active material having high electric capacity and being excellent in thermal stability and storage characteristics in an environment having high temperatures can be obtained. However, a positive electrode active material which satisfies requirements for high electric capacity and high output has not yet been obtained.
On the other hand, in order to improve output characteristics, a method for adding a tungsten compound to a lithium-nickel composite oxide has been examined.
For example, Patent Literature 3 proposes a positive electrode active material for a non-aqueous electrolyte secondary battery containing a lithium metal composite oxide powder including a primary particle represented by the general formula: LizNi1-x-yCoxMyO2 in which 0.10≤x≤0.35, 0≤y≤0.35, 0.97≤z≤1.20, and M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti and Al, and a secondary particle including an aggregate of the primary particles, in which the surface of the primary particle has a fine particle containing W and Li.
Although the positive electrode active material is improved in output characteristics, the content of nickel in the positive electrode active material is low. Therefore, increase of an electric capacity has been desired for the positive electrode active material. In addition, when the content of nickel in the positive electrode active material is increased, there is a necessity to examine thermal stability of the positive electrode active material.
As mentioned above, although various lithium-nickel composite oxides in which a part of nickel is substituted with other element have been developed, a positive electrode active material made of the lithium-nickel composite oxide which can sufficiently respond to the requirements for high electric capacity and high output when used in a non-aqueous electrolyte secondary battery has not yet been produced.